Included among the many considerations in designing apparatus for vertical slab gel electrophoresis is the need to load samples in a reliable and reproducible manner, and in such a manner that a maximum number of samples can be separated simultaneously in a single gel, each with sharp band resolution.
Slab gels are usually formed by joining two glass plates together with spacers at both vertical edges to establish a gap between the plates which usually measures 0.25-3 mm in depth. The plates are clamped together along the vertical edges and a seal is placed along the open bottom edge. The gap is then filled with gel solution, and the gel is allowed to set. In some slab gels, a well-forming insert, referred to in the industry as a "comb" or a "template," is placed along the open upper edge of the gel solution before the gel sets, the teeth of the comb extending into the space between the plates. After the gel sets, the comb is removed to leave a row of wells formed in the gel along its top edge for example loading.
In other slab gels, particularly those ranging from 0.25-0.40 mm in thickness, a different type of comb is used, one which is placed between the gel plates after the gel has set and left in place during the electrophoresis. This comb is commonly referred to as a "sharkstooth comb" since its teeth are pointed and longer than those of the combs described above. Once the gel has been set, the comb is inserted between the plates such that the tips of the comb's teeth contact the upper edge of the gel. Between the teeth of the comb are inverted U-shaped spaces, each of which is used as a well to receive a sample which is to be subjected to an electrophoretic separation.
In structures where sharkstooth combs are used, the plates which hold the gel differ in height, with one plate extending a short distance above the other. The upper edge of the gel is located a short distance below the upper edge of the shorter plate. The sharkstooth comb is inserted between the plates in such a manner that the U-shaped spaces between the teeth extend above the shorter plate but not the taller plate, which closes them on one side. Liquid sample aliquots are then introduced into the spaces at their open sides, flowing into the area between the plates above the gel, where they form individual pools separated by the teeth. Since the teeth are narrow at their pointed ends, the samples although separated are very close together, forming essentially contiguous lanes.
The gels in which sharkstooth combs are used are generally very thin, and it is important that their thickness be uniform, since variations in thickness will affect the migration rates of the sample components and band resolution, and thus reproducibility as well as the ability of the results to be read and interpreted properly. The sharkstooth combs are likewise very thin, and inserting such a comb between glass plates which are so close together is a delicate procedure. The thinness of the combs as well as their sharply pointed teeth make the combs particularly delicate and susceptible to breakage.